The present invention relates to a photomask inspecting method and apparatus.
Recently, in order to increase the integration degree of semiconductor integrated circuits, various micropatterning techniques have been proposed. In general, lithography is used for patterning. In order to form micropatterns, the resolution power of a reduction projection exposure apparatus (called a stepper) used for lithography must be increased. A technique of increasing the numerical aperture of a projection lens and decreasing the wavelength of a light source has been employed to increase the resolution power of this reduction projection exposure apparatus. This technique, however, has already achieved a resolution power close to its theoretical limit. For this reason, studies have recently been made to use other techniques to increase the resolution power.
Methods of overcoming the above theoretical limit include a method of giving a phase difference of nearly 180.degree. to light passing through a transparent portion of a patterned photomask in processing the photomask, and a method of giving a certain degree of transparency and a phase change to a pattern portion. These photomasks are generally called phase shift masks. Such a method is disclosed in, e.g., J4 of Proceedings of The 36th International Symposium on Electron, Ion and Photon Beams.
This technique is a technique of improving a light intensity distribution on an image surface as a result of interference of light from each portion of a photomask. A phase shift technique will be described below in comparison with a technique using a normal photomask.
FIG. 26A shows amplitude and light intensity distributions obtained by performing image formation using a normal photomask NPM using a line-and-space pattern formed by selectively placing mask members on a glass substrate. In the amplitude distribution characteristics shown in FIG. 26A, the dotted lines represent the amplitude distribution of transmitted light from the respective spaces, and the solid line represents the amplitude distribution as a result of interference. The light intensity distribution is obtained by squaring the amplitude distribution represented by the solid line.
FIG. 26B shows amplitude and light intensity distributions obtained by using a phase shift photomask PPM. Similar to FIG. 26A, the dotted lines represent the amplitude distribution of transmitted light from the respective spaces, and the solid line represents an amplitude distribution as a result of interference. The light intensity distribution is obtained by squaring the amplitude distribution represented by the solid line.
In the normal photomask NPM shown in FIG. 26A, since diffracted light beams from adjacent transparent portions are superposed in phase, the light intensity of each light-shielding portion does not become 0. In contrast to this, in the phase shift photomask PPM shown in FIG. 26B, since diffracted light beams from adjacent transparent portions are superposed in opposite phases, the light intensity of each light-shielding portion becomes 0. As a result, with the phase shift photomask PPM shown in FIG. 26B, the contrast of an image is improved. Note that the contrast improving effect is reduced as the phases of light beams from adjacent transparent portions shift from .pi., and the amplitude of light transmitted through a phase shift member decreases because of absorption in the phase shift member.
In a phase shift mask, therefore, it is very important to control the phase and amplitude transmittance of transmitted light from each portion. If the refractive index and extinction coefficient of each material used for a photomask are accurately obtained, a phase, an amplitude transmittance, and an energy transmittance can be controlled by controlling the thickness of each material.
In many cases, however, it is difficult to accurately measure the refractive index and extinction coefficient of each material. In addition, impurities are mixed in each material in the manufacturing process to cause errors in the refractive index and the extinction coefficient. Furthermore, the thickness of each material is inevitably accompanied by a manufacturing tolerance.
For this reason, a phase, an amplitude transmittance, and an energy transmittance are actually measured with respect to each portion of a manufactured photomask, and the results are fed back to the manufacturing process. These operations are repeated to realize a desired photomask. However, no effective means for measuring a phase, an amplitude transmittance, and an energy transmittance is available.